It has heretofore been known, for example, from Japanese Patent Publn. No. 7875/1973 that imide prepolymers are prepared by reacting bisimide with diamine in a solution state by using a high boiling solvent such as N-methylpyrrolidone, or heating the above-mentioned reactants to yield a molten mixture which is allowed to undergo reaction.
When the imide prepolymers thus obtained are cured by heating, cured products excellent in heat resistance are obtained. However, the cured products obtained have such a problem that they are brittle. In addition, they were not always satisfactory in point of tensile strength, tensile elongation or Izod impact strength.
When a laminate sheet is prepared by impregnating a glass cloth with a laminating varnish comprising this prepolymer, there is such a problem that the prepolymer exhibits only a low adhesive strength to glass cloths or metals such as copper.
Further, when this prepolymer is dissolved in polar solvents such as dimethylformamide, N-methylpyrrolidone and dimethylacetamide, a fairly large amount of insoluble portion thereof is left behind, and the insoluble portion rapidly increases in amount with increasing softening point of the prepolymer. On that account, there is such a problem that the preparation in the presence of the above-mentioned polar solvents of imide prepolymers having a high softening point involves difficulty.
Under these circumstances, the present inventors prosecuted extensive researches, in addition to unsaturated bisimides and diamines which are the starting materials for imide prepolymers, on the addition of radical polymerization inhibitors, carboxylic acids or acid anhydrides to the reaction system, and on the kind and mixing ratio thereof, and on the reaction conditions to be employed therefor, and eventually have accomplished the present invention on the basis of a finding that the above-mentioned problems can be solved at a single stroke by using at least one specific radical polymerization inhibitor in specific proportions.